1. Field of the Invention
The invention relates to fluid pumps, and in particular to gear pumps and other positive displacement hydraulic pumps, which can be used to deliver hydraulic fluid to two different sets of hydraulic loads. A priority valve is needed to distinguish between the two loads, and deliver hydraulic fluid preferentially to a first load up to a first working pressure, and only then deliver hydraulic fluid to the second load which is a non-preferential load.
2. Description of the Related Art
Priority valves are known which divide the flow from a hydraulic pump into preferred and non-preferred flows for servicing two loads as indicated above. The majority of such priority valves have been connected in series with the pump output, being connected to the pump by conduits and to the first and second loads by further conduits.
In GB 2298902, the present Applicant discloses a pump incorporating an integral priority pressure regulating valve. The valve is spring biased at one end face in a direction to permit fluid communication between a high pressure chamber of the pump and a main port connected to the preferential load only. An opposing end face of the valve is supplied with hydraulic fluid from the main port in such a manner to counter the spring bias. When the main port receives hydraulic fluid of a predetermined pressure, the pressure on the opposing end face is sufficient to overcome both the compressive force developed by the spring and the static friction associated with the valve, thereby enabling the movement of the valve to a position where it permits fluid communication between the high pressure chamber and an auxiliary port which is connected to the non-preferential load. This pump is much simpler to install than one requiring a separate priority pressure regulating valve to be inserted in the pipeline or conduit between the pump and the main and auxiliary loads, and there is a much lesser tendency for fluid leakage.
In this arrangement, the spring provided to bias the valve must be of sufficient strength so as to meet the total reaction developed against it when fluid of the predetermined pressure is applied to the non-spring end face of the valve. Quite often, the predetermined pressure selected is relatively high and hence the loading on the spring can be excessive.
The characteristics of the spring are extremely important since the spring must be compressed to a depth equal to the length through which the valve is require to travel without exhibiting substantial changes in its reaction against the valve, otherwise the reaction developed by the spring against the valve will increase significantly as the spring is compressed.
Furthermore, whereas pressure is uniformly applied to the non-spring end face of the valve from the main port, the force exerted by the spring on the valve is localized through the points of contact between the spring and the valve. This may induce distortion of the valve profile.
Additionally, as the spring is an integral component to the pump, it is a relatively difficult operation to adjust or replace the spring so as to provide the pump with a new predetermined pressure setting.
Therefore, it is an objective of the present invention to significantly reduce the problems identified above in relation to the prior art. This is achieved by means of pilot operation.
The invention provides a fluid pump having a housing, a main output port, an auxiliary output port and a priority pressure regulating valve contained within the pump housing. The priority pressure regulating valve includes a spool having two opposing end faces. Each of these end faces is disposed within a chamber which is in fluid communication with the main output port. A force means is also included in association with one of the spool end faces to bias the spool to a position where it causes fluid developed by the pump to flow to the main output port exclusively. A pressure release means is provided in fluid communication with one of the chambers to enable fluid to flow from said one of the chambers when the pressure at the main output port is at a predetermined working pressure thereby establishing a pressure differential across the two end faces of the spool which is sufficient to overcome the bias developed by the force means thus causing the spool to move to a position where it permits fluid developed by the pump to flow to the auxiliary output port.
In a preferred embodiment, that chamber which is in fluid communication with the pressure release means houses the force means. In this arrangement, the force means is preferably a coil spring in compression.
Alternatively, the force means may be provided in the chamber which is remote from that which is in fluid communication with the pressure release means. In these circumstances, the force means may be a coil spring in tension.
Preferably, the pressure release means includes a poppet, a regulating spring and an adjuster, wherein one face of the poppet is in fluid communication with one of the chambers and the regulating spring is disposed to resist the force exerted on the face of the poppet by the pressurized fluid contained in said chamber. The poppet may be located between said chamber and a channel such that when the pressure exerted on the poppet by the fluid in said chamber overcomes the opposing force exerted on the poppet by the regulating spring, fluid communication is established between said chamber and the channel.
In a preferred embodiment of the invention, the channel drains to an inlet of the pump.
The regulating spring may be a helical spring in compression which engages that face of the poppet which opposes the face which is in fluid communication with said chamber. Additionally, that end of the regulating spring which is remote from the poppet may abut the adjuster in a manner such that the adjuster can be moved along the axis of the regulating spring to adjust the compressive force exerted on the poppet by the regulating spring.